After the battle against the "All-devouring Narwhal", you wanted to see how Childe's doing. Arlecchino told you, he was merely injured and recovering, but she left out one detail... Childe was partly "stuck" in his Delusion form, he never returned to Snezhnaya and was hiding somewhere in Fontaine... You received a commission a few days later, to find a certain "monster" in Fontaines area. That was when you saw him and his pained expression...